Communication system



Patented Nov. 2, 1943 COMMUNICATION SYSTEM Clarence W. Hansell, Port Jefferson, N. Y., assignor to Radio Corporation of America, a corporation of Delaware Application February 7, 1941, Serial No. 377,809

6 Claims.

This invention relates to communication sys-' tems and. more particularly, to improved apparatus and methods of operation for reducing multi-path distortions in modulated carrier wave communication systems operated in such a manner that differences in time of travel of waves arriving over dierent paths becomes comparable with the time of a half cycle of the highest modulation frequency. v

In ultra high frequency communication involving television and other services requiring wide modulation frequency bands, and particularly in urban areas where there are many high buildings one of the fundamental obstacles to satisfactory service is reception f waves arriving from the transmitter over secondary paths caused by reflection. These secondary paths are different in length from the direct path and the carrier wave and modulation wave components received over the different paths do not have like phase relations or timing and, consequently, ghost signals in addition to the main modulating signal and of the carrierl signal are produced by the interaction of waves received over .the main path and secondary paths. The time delay of modulations received over some of the secondary paths is so great andvtheir energy so great that one or more of the secondary signals often cause serious interference and, in the case of the transmission of television signals, one or more secondary images are often plainly visible and the image at the television receiver, instead of being a good reproduction of the scene at the transmitter, appears as a multiple exposed photograph taken with a camera which is moved between exposures.

Various methods and means have heretofore been proposed to o bviate the appearance of such ghost signals. It has been found that, by operating two or more entirely independent transmitters in different frequency bands but modulated simultaneously, the signals received are not subject to the same detail distortions although-- `render 'such a system impractical for many applications.

According to the present invention, a diversity effect is obtained by using a single transmitter in which the carrier frequency is changed from one frequency to another, periodically between repetitions of portions of the amplitude-modulating signals. The invention is applicable to a'system employing any kind of modulation, including am.- plitude, phase, frequency and single side band modulation systems.

The primary object ofthis invention is to reduce multipath distortions in modulated carrier wave communication.

Another object of this invention is to reduce the .objectionable ghost images -in television reception.

Other and incidental objects of the invention will be. apparent to those skilled in the art from a reading of the following specification, considered in connection with the .accompanying drawing, in which Figure 1 is a block diagram showing oneform of this invention, and I Figure 2 is a block diagram showing another form of this invention as applied to the transmission of television or facsimile signals.

Referring now in more detail to Fig. 1, a signal including synchronizing pulses'is applied to the input terminals I and 3. A portion of this signal, after amplification, is applied to the trans mitter 1 which may be of any of the types known to the communication art including amplitude' phase, frequency and vestigial or single side band types. carrier wave output to the antenna, not shown.

Another portion ofthe input signal is segregated by the unit 9 which, in, the case of television signals, will be the vertical or the horlzontal synchronizing pulse separator and used to control the frequency of the transmitter 'I through tripping circuit l I and frequency modulator I3. f Any of the pulse separators well known in the art may be used, for example, one such as shown and described. in Toison Patent No. 2,207,839, issued July 16, 1940; The pulse controlled tripping circuit may be one such as shown and de.- scribed in Andrew Patentv No. 2,000,685, issued May 7, 1935. 'A frequency modulator or frequency control circuit such as shown in Travis Patent No. 2,155,208 of April 18, 1939 may be utilized to control the frequency of the transmitter 1.

Undercontrol of pulses from synchronizing l The transmitter 1 provides modulated pulse separator amplifier 9, the pulse controlled tripping circuit II supplies rectangular wave form potential or current to modulator I 3. As a result. one repetition of modulation input is transmitted with one value of transmitter carrier frequency, and the next repetition is transmitted with another value of transmitter carrier frequency. In television, the transmitter frequency is shifted from one value to another during bianking periods betweenpassages over the image.

Since successive repetitions are transmitted with different carrier frequencies, the multi-path distortions will not be alike on` successive repetitions, but the useful modulations will be..

Therefore, a better ratio of image modulation to distortion modulation will appear on reproduced images inthe output from a receiver.

It may be noted that when the carrier frequency is shifted between scanning lines in a television system, and an odd number of lines per frame are used, then successive repetitions of the lines in the image will be done with different frequencies so that the reduction in distortion will be effective in each line as well as in the image as a whole.

Fig. 2 shows a particular form of this invention as applied to an amplitude modulated television transmission system in which synchronizing signals are transmitted periodically. A synchronizing frequency signal vgenerator I5 applies a synchronizing signal to a deiiecting signal generator II which, in turn, generates a deflection voltage which is adapted to control the rate of scanning ofthe scanning device I 9.

Any of the signal generators and deflection signal generators, such as, for example, the one shown and described in the Toison etal. Patent No. 2,101,520, may be used. A suitable scanning device is shown and described in a paper entitled Television Pickup Tubes with Cathode Ray Beam Scanning" by Iams and Rose `in theProceedings of the Institute of Radio Engineers for August 1937.

The output signal from the scanning device I9 is applied to an amplitude modulator 2i designed to modulate the carrier wave output of the radio frequency amplifier 23.

A portion of the signal output from the synchronizing frequency signal generator I5 is vapplied to the square wave generator 25, which changes the frequency of the oscillator 29 by a predetermined amount through a frequency modulator amplifier system 21.

In. the operation of this invention as applied to television signal transmission, the signals may be reproduced by any of the well known television receivers, such as, for example, the one shown "and described in the Vance Patent No. 2,137,039, issued November 15, 1938. vIt is proposed to provide circuits at the Vtransmitter by means of which the carrier frequency may be switched from one value to another synchronously with scanning of the image or synchronously with the line scanning. According tocertain television standards in the United States, generally referred to as interlaced scanning, images will be scanned at the rate of sixty elds per second, taking each alternate line of the scanning pattern for each transversal of the picture. According to this standard, for example, the whole picture is scanned at a' rate of thirty fields per second. In the operation of one form of this invention as applied to television transmission, Vthe carrier frequency is switched from one value to another between elds at a rate of sixty times per second.

It, therefore, follows that alternate lines in the image will be subject to different multi-path distortion effects, depending upon the characteristics of the space circuits for the two different carrier frequencies.

Consequently, the overall distortion of the television image, as observed by the eye, will be similarly located by the two sets of alternate lines in the Areproduction on the picture screen and will have the same polarities, whereas the'undesired or secondary images will appear with almost random selection of positive or negative images andfin many cases, a positive secondary image on one set of alternate lines will appear as a negative secondary image on the other set of alternate lines'. Thusl the secondary images in some cases will be made to add their effects very imperfectly or even, at some times, to oppose their effects and they will therefore be less troublesome to theobserver. In a similar` manner, distortion may be reduced by switching from one frequency to another at a rate of 30 or 15 times per second. Likewise, the switching may be done at the rate of line repetition or, say, 13,230 times per second for 441 lines per frame, 30 frames per second television.

To illustrate the operation of one form of this invention, we may assume, foi' example, a television system 'in which there are two paths by Whichenergy travels from transmitting to receiving antenna. We may further assume that one of these paths is a main path over which currents of amplitude reach the receiver, while the other is a secondary path over which currents of 10% amplitude reach the receiver. Let it further be assumed that the carrier frequency, to begin with, is 50 .megacycles per second correspending to a wave length of 6 meters and 4that the two space circuit paths differ in effective length by 600 meters or 100 Wave lengths.

For the specific conditions assumed, every modulation of the carrier wave received over the main path will be followed polarity and with 10% amplitude.

If the frequency of the carrier is slowly varied in one direction, starting with .the conditions assumed, it would be found that the strength of the secondary modulation would decrease to a minimum,'become negative, return'to a minimum, and then to a positive, etc., going through a complete cycle each time the frequency was changed enough to change the number of waves by one in the distance equal to the ldifference between the two path lengths. To obtain an exactly 'opposite secondary modulation at the receiver, it is necessary to change the frequency enough to change Ipath length difference by .5 wave lengths. Since we assume the differences in path lengths to be 100 wave lengths and the the secondary path modulation would be reversed.

For this particular assumed condition it would be preferable to transmit one set of alternate lines #49,751,244 cycles per second proportion to the frequency.

of a television image of a carrier frequency of 50 megacycles per second followed by another set of alternate lines with a carrier frequency of either 50,250,000 or 49,751,244 cycles per second. Then the images produced by the two sets of lines would have equal and opposite multi-path distortions and tend to balance out their effects upon the eye of the observer.

In practice, it is unlikely that we might have exactly the conditions assumed in this case but, considering the effects at many receivers, there may be many conditions of time delay and random phase relations between the resultant carrier and secondary path modulations. There may also be a considerable number of secondary paths contributing to the multi-path distortions of the image. Therefore, the invention cannot be expected to eliminate all multiple images as has been done in the example given, but there may be expected a considerable average reduction of ghost images. The main path modulations add their effects nearly perfectly in the reproduced image but the secondary path modulations usually add imperfectly or oppose one another and so have a reduced effect in distorting the image. The calculation of the amount of reduction in multiple image distortion in the average case indicates that a definite improvement in the resultant image will be obtained.

A most decided'improvement in the results obtained by the employment of this invention in point to point communication or relay systems may be observed when the system is adapted to use only one receiver at a fixed location with respect to the transmitter. In such a case, the frequency shift may be adjusted to obtain a result in accordance with the above example, whereby the principal interfering ghost signal may b substantially completely eliminated.

The optimum spacing between the two carrier wave frequencies in the preferred arrangement of the present invention is a matter of experience and judgment which had best be determined by trial. In general, the greater the difference between the two carrier frequencies, the more nearly will the multi-path distortions be reduced to a minimum. However, wide spacing is objectionable because of loss of frequency band width for other transmitters, because of loss of signal to noise ratio and because of design problems introduced through the necessity for using wider transmitter and receiver frequency pass bands.

Any number of carrier frequencies may be used, as for example, four carrier frequencies, and in such a case. it is preferable to change inthe order 1, 3, 2, 4, l, 3, etc. with changes occurring between repeating modulations or image scanning.

It should be understood that the amount offrequency shift between repetitions, required to Bain the maximum benefit from this invention.l is a function of the difference in time delay over the direct and indirect paths and is independent Vof the frequency range in which, the transmitter is operating. As a consequence, the required percentage change in carrier frequency is in inverse At 500 megacycles. the percentage frequency shift between repetitions may be only one-'tenth as great as at 50 megacycles. Therefore, the invention can be applied more and more readily as the frequency is increased.

The invention is applicable not only to overcoming multi-path in radio on systems, but also' is applicable to wire line and cable commimication systems and to supersonic wave communication systems such as are used principally for submarine signalling.

While several systems for carrying this invention into effect have been indicated anddescribed, it will be apparent to one skilled in the art that this invention is by no means limited to the particular organization shown and described, but that many modifications may be made without departing from the scope of this invention as set forth in the appended claims.

I claim as my invention:

1. 'I'he method of reducing multipath distortions in modulated carrier wave communication systems comprising the steps of periodically selecting successive portions of said carrier wave modulations and changing the carrier wave frequency between each portion by an amount necessary to change the path length difference in wave lengths by substantially one half wave length.

2. The method of reducing multipathdistortions in modulated carrier wave communication systems comprising the steps of periodically selecting successive portions of said carrier wave modulations and changing the carrier wave frequency at a rate equal to said selection rate by an amount such that the difference in the num ber of wave lengths in the primary and a, secondary path varies by substantially one half wave length.

3. In a communication system of the type wherein at a predetermined carrier frequency there is a primary path Aand an interfering secondary path caused by reection. the method of reducing the interference caused by said secondary path comprising the steps of periodically changing the carrier frequency between two predetermined values which differ by an amount which changes said path length diiference by substantially one half wave length.

4. In the operation of a carrier wave cornmunication system including a main signal path and at leastone reflection path which tends to produce undesired images of the signal, the method of reducing undesired images which includes periodically changing the frequency of said carrier between predetermined values which differ by the amount required to change the number of half wave lengths in the path length difference by one half wave length.

5. 'I'he method of reducing multipath distortions in modulated carrier wave communication systems comprising the steps of amplitude modulating a carrier wave with a signal which periodically repeats at a predetermined rate, generating a square wave signal whose frequency is onehalf of said predetermined rate, and using said square wave signal to frequency modulate said oscillator by an amount which changes by one the number of half wave lengthsin the diiference between the primary and a secondary path length.

6. In a' carrier wave communication system ofV the type including the transmission of periodically reoccurrlng synchronizing signals and wherein at a predetermined carrier frequency there isa primary transmission path and an interfering secondary transmission path, the

'method of reducing the interference caused by 

